The film obtainable from EVOH is transparent and excellent in oxygen-barrier property and, therefore, laminates of EVOH film with other plastic film, paper, metal foil, etc. are useful packing materials for foodstuffs, pharmaceutical products, industrial chemicals and so on.
Any desired technique available can be employed for forming an EVOH film on a substrate but the solution coating method comprising coating the surface of a substrate with a solvent solution of EVOH is attracting attention because a comparatively thick film can be formed, the film can be easily formed even on a substrate having a complicated profile, such as a hollow vessel, and the coating can be performed with a comparatively simple apparatus.
However, a solution obtainable by dissolving the ordinary EVOH in a solvent mixture of water and alcohol has a poor storage stability, i.e. a short pot life, and when stored at room temperature, becomes opacified in a few days and ultimately solidifies. Therefore, it is poor in handlability, requiring a field operation, such as redissolving EVOH by heating the solution at 60.degree.-80.degree. C. for a few hours, for instance, immediately before use. Furthermore, this EVOH solution is so high in viscosity that it is inferior in high-speed coatability.
For improving the stability of an EVOH solution, a method comprising adding boric acid to EVOH has been proposed (Kokai Tokkyo Koho H-2-47144) but even with such contrivances the storage stability cannot be said to be sufficient. Moreover, because of the high viscosity, the high-speed coatability is not improved at all.
Thus, the conventional ordinary EVOH solution or the solution prepared by adding boric acid thereto is unsuitable as a coating.
Kokai Tokkyo Koho H-4-500834 describes the use, in a polymer composition for the production of a biodegradable plastic product made up of a modified starch and a synthetic polymer comprising EVOH or polyvinyl alcohol, of a polymer oxidized by reaction with hydrogen peroxide and persulfuric acid as said polymer.
However, when EVOH is dissolved in a solvent and oxidation-modified by the reaction with hydrogen peroxide and persulfuric acid, the problem remains that the resulting oxidation-treated polymer solution becomes colorized, its standing stability is adversely affected, and the gas-barrier property of the coating film is sacrified.